As a reminder, FIG. 1 illustrates a phase of acquiring data by a mobile carrier in “Spot” SAR mode, that is to say a mode in which the antenna beam is slaved permanently to the zone to be imaged. A radar fixed to an aircraft 101 illuminates an imaged zone 102 for a duration of illumination Te by slaving the antenna beam 103 onto the center 104 of said zone 102 along the whole of the trajectory 105 of the aircraft 101. This duration Te is inversely proportional to the resolution aimed at on the transverse axis 106, the resolution on the radial axis 108 being for its part, inversely proportional to the band emitted by the radar antenna. The imaged zone 102 is meshed by a net 110 of cells for each of which it is sought to associate at least one reflectivity level.
The detections of the radar make it possible to create an image on the radial axis 108 and the transverse axis 106, which are designated subsequently by the terms “Distance” axis 108 and “Doppler” axis 106 respectively. This image, referred to hereinafter as the “Distance-Doppler” image, delivers for each cell M situated inside the imaged zone 102, a distance value DM and a Doppler frequency value fM, these two values DM and fM being referenced with respect to a given instant tref corresponding, for example, to the elapsing of half the total duration of the illumination.
By describing a given angular sector around the imaged zone 102, the radar periodically collects a series of N distance-wise profiles with a recurrence frequency fr equal to N/Te. Each of the N distance-wise profiles offers a one-dimensional representation of the imaged zone 102 along the distance axis 108. Furthermore, the distance axis 108 is divided into several bins, each of said bins preferably having a size which is slightly smaller than the distance-wise resolution. For a given distance bin, a spectral analysis along the transverse axis 106 performed on the collected signal makes it possible to discriminate Doppler-wise the various echoes contained inside this bin. This spectral analysis makes it possible to discriminate the echoes with the desired resolution if certain conditions are satisfied. To satisfy these conditions, focusing algorithms apply corrections to the signal collected for each of the reflectors of the imaged zone 102, these corrections comprising:                on the one hand, a distance-wise migration correction, to compensate for the variation in distance between the reflector and the phase center of the antenna in the course of the illumination;        on the other hand, a Doppler-wise migration correction, to compensate for the non-linear phase term of the signal due to the variations in the radar-reflector closing speed, so as to preserve a fixed-frequency signal.        
The use of conventional focusing algorithms makes it necessary to know very accurately the trajectory of the phase center of the radar antenna in the course of the acquisition of the signals, a fortiori when the desired image resolution is fine, the required illumination time for the imaged zone then being long, this illumination time possibly, for example, exceeding a minute. Now, when the radar antenna is fixed to a mobile carrier such as an airplane, which is particularly sensitive to atmospheric disturbances, this trajectory cannot generally be known with sufficient accuracy, especially when the radar does not possess any inherent inertial system. Hence, so-called autofocus algorithms correct the residual focusing defects by utilizing the information contained in the radar signal itself, without using solely the outside information regarding trajectory measurement.
To summarize, conventionally, the generation of a focused SAR image proceeds in three steps. In a first stage, a full-resolution image is generated by a known image formation algorithm: there is then still a residual defocusing on account of the inaccuracies in measurement of the trajectory of the phase center of the antenna, stated otherwise, on account of the inaccuracies in measurement of the trajectory of the carrier. In a second stage, this residual defocusing is estimated by an autofocus technique applied to the previously computed “full-resolution” image. In a third stage, the image generated in the first step is re-focused using the previously estimated residual defocusing. This approach includes several drawbacks.
On the one hand, it totally dissociates the first step from the following steps. Now, the first step relates to the formation of a full-resolution image, the focusing quality of which depends on the quality of measurement of the trajectory of the phase center of the antenna, which measurement is generally performed by a sensor external to the radar, while the following steps endeavor to perform autofocusing processing operations utilizing the previously formed full-resolution image. Thus, when the illumination times become very long, errors measuring the trajectory of the carrier cause a deterioration in the focusing quality such that, on completion of the first step, the residual defocusing becomes impossible to estimate correctly in the course of the second step.
On the other hand, this approach makes it necessary to wait for the end of the acquisition of the radar signals before beginning to form the image since the first step is aimed at the formation from the outset of an image of finest possible resolution by utilizing the whole of the acquisition. Now, it is sometimes useful for roughly resolved images of a given zone to be available before the illumination time for said zone has elapsed entirely. Likewise, in a concern to optimize the use of the tools for processing radar signals, it may be advantageous to perform real-time computations, without waiting for the end of the complete illumination of the zone. The SAR image construction methods currently employed do not make it possible to satisfy these requirements in a simple manner.
Finally, the SAR images are polluted inherently by multiplicative noise which may be detrimental to the readability of the image. An effective scheme for solving this problem is to construct several images by rotating about the imaged zone so as to sum these images in terms of power after having superimposed them suitably, so that the standard deviation of the noise is reduced. However, the known methods of SAR imaging do not generally incorporate this scheme naturally during the generation of the image.